Developing Thermal Models for Power Transformers

Developing Thermal Models for Power Transformers using CFD Hugo Campelo, EFACEC Energia, R&D Department Porto, Portugal @ Ansys Users Conference, 10th November 2011, Portugal

22 Janeiro 2010

SUMMARY

1st

CFD Simulation in Shell-Type Transformers

Experimental Validation

2nd

CFD Simulation in Core-Type Transformers


GOAL

FluCORE – Analytical tool to predict temperatures


POWER TRANSFORMERS? 10-40kV TOP-DOWN STYLE GRID

220kV 400kV

70kV 220V

IS IT CHANGING TO AN INTERNET STYLE GRID?

EFACEC HISTORY

1947 - Foundation of Empresa Fabril de Máquinas e Equipamentos Eléctricos (EFME) Produced only electrical motors.

20% Electro-Moderna 20% ACEC 45% CUF 15% Other Shareholders

EFACEC HISTORY 1957 – First Power Transformer Produced

1969 – Lisbon Stock Exchange.

1997 – Build a power transformer plant in China.

2005 – Out of Lisbon Stock Exchange. Public Take Over Bid (TOB). 50% Grupo Mello (CUF)

50% Têxteis Manuel Gonçalves (TMG) 2009 – Build a power transformer plant in USA, Georgia.

POWER TRANSFORMERS?

SHELL-TYPE

Flux-return path external to windings

Better magnetic shielding

CORE-TYPE

Flux-return path internal to the windings

POWER TRANSFORMERS? CORE-Type Transformers Up to 350 MVA and 400 kV (BIL 1425 kV)

POWER TRANSFORMERS? SHELL-Type Transformers Up to 1500 MVA and 525 kV (BIL 1675 kV)

Typical location would be, for example, a Nuclear Power Plant.

POWER TRANSFORMERS?

PRIMARY

Magnetic Field

SECONDARY

Magnetic Circuit Energy SECONDARY ≈ 99 % * Energy PRIMARY

Joule Effect Foucault Currents

Typically the remaining energy, under heat form, is removed by circulating mineral oil inside the transfomer.

To avoid overheating.

CFD SHELL-TYPE [1]

-CFD Software Fluent 6.3.26 and GAMBIT 2.3.16 -Parallel processing

Finite Element Volume mesh ~1 000 000 Cells

[1] P. Gomes et al. “Large Power Transformer Cooling – Flow Simulation and PIV analysis in an Experimental Prototype”, ARWtr2007.

CFD SHELL-TYPE Outlet

Location

Boundary Condition

Description

Inlet

Pressure Inlet

Pressure (Pa)

11 800

Material

Oil

Theoretical Viscosity (kg.m-1.s-1)

0.0217

Density (kg/m3)

868

Pressure (Pa)

0

Interior

Outlet

Fluid

Pressure Outlet

Spacers Symmetry

- 3D/2D Simulations - Steady State - Laminar Flow 6 5 4 3 1 2

CFD SHELL-TYPE Oil Velocity

111 cm/s - Non-uniform oil distribution; - Low velocities in top /bottom inner zones;

0 cm/s

Contours of Velocity Magnitude

CFD SHELL-TYPE Washer Inlet Area %

-Inlet and Outlet oil flow rate not proportional to zone area;

Flow Rate (%)

- Oil enters by the right side of the washer; - Maximum flow rate is 20 times higher than minimum; Inlet Zones

- Static Pressure not uniform for curved zone.

11.8 kPa

Oil Stream Lines

Static Pressure Contours

0 kPa

CFD SHELL-TYPE Washer Centre

-“Zig-Zag” Oil Flow; 115 cm/s

- Stagnation Zones after Spacers;

0 cm/s Oil Velocity Vectors

Oil Stream Lines

CFD SHELL-TYPE Different Pressure Inlets / Flow rates 11.8 kPa

1.18 kPa

0.118 kPa

110 cm/s

15 cm/s

2 cm/s

0 cm/s

0 cm/s

0 cm/s

Contours of Oil Velocity Magnitude

- Oil flow pattern independent of flow inertia;

CFD SHELL-TYPE Alternative Design Concept

- Optimal spacer offset; 0.6 L L

1.3 L

- Oil Flux area = 63%; - Non-supported copper distance of 60mm

L = 30mm

CFD SHELL-TYPE Oil Velocity

110 cm/s

0 cm/s

Original Design

Alternative Design

CFD SHELL-TYPE Washer Inlet Area % Flow Rate (%)

Inlet Zones

Original Design

Alternative Design

1

Flow Rate (30% increase);

2

Homogeneous oil flow;

3

Stagnation zones size decrease 70%.

CFD SHELL-TYPE Washer Inlet

1.18 kPa

0 kPa

Original Design

Alternative Design

Static Pressure Contours - Static Pressure Uniformity for washer curved zones;

CFD SHELL-TYPE – Exp. Validation

Camera

Image A instant t

Image B instant t+Δt


Image A, instant t

Dx

Velocity = Δx/ Δt

Image B, instant t+Δt

CFD SHELL-TYPE – Exp. Validation • Experimental Set-up

Top Tank

Bottom Tank Pump

ACRYLIC Washer to be Laser Transparent.

CFD SHELL-TYPE – Exp. Validation Experimental Set-up

Laser parallel to washer plane

Camera perpendicular to washer plane

TSI laser and Camera

CFD SHELL-TYPE – Exp. Validation 60 cm/s

0 cm/s

Experimental Field

CFD Field


Experimental Field

CFD Field

CFD SHELL-TYPE – Exp. Validation Experimental 40 cm/s

0 cm/s

Good agreement between experiments and CFD.

CFD

CFD CORE-TYPE

Symmetry Symmetry

[2] H. Campelo et al., “Detailed CFD Analysis of ODAF Power Transformer”, Cigré HRO, 2009.

CFD CORE-TYPE Temperature (ºC)

2D/3D Simulations ONAN/ODAF Regimes

Mesh resolution effect Modified BC EQ. THERMAL CONDUCTIVITY

Oil radial velocity (m/s)

CFD CORE-TYPE

Axial coordinate (m)

CFD CORE-TYPE

Disc temperature (ºC)

Detailed conductors

Detailed Conductors Temperature (ºC)

Simplified conductors

Appx. -2ºC

Simplified Conductors

Radial Disc Coordinate (from interior to exterior) in m

CFD CORE-TYPE – Exp. Validation 60/15.75 kV, 20 MVA, Heat Run Test in ONAF Regime 4 Optical Fibres (Oil + Windings) Location

Temperature (ºC)

Winding

Disc#

Azimuthal Measured (±1.7˚C)

HV

2-3

spacer

115.8

99.1 - 115.1

LV

2-3

spacer

108.9

90.8 – 103.7

HV

3-4

oil

97.7

91.0 - 115.5

LV

3-4

oil

97.9

89.8 - 99.6

Good agreement between CFD results and measurements.

CFD

CFD is great! Shall I use it to design EVERY Power Transformer? NO…FOR SEVERAL REASONS: 1 Computational effort is huge; N-1

Power Transformers Designers have minutes not days;

Build a SIMPLIFIED/ANALYTICAL tool based on CFD which is now an APPROVED NUMERICAL LAB.

FluCORE CONCEPT → Hydraulic - Electrical Analogy

Kirchoff’s nodal rule

qk  qk  2  (qk 3  qk 1 )  0 ~

~ Kirchoff’s loops rule ( pk  pk  2 )b  ( pk  2  pk 3 ) rad  ( pk 1  pk 3 ) c  ( pk  pk 1 ) rad  0

~

~

FluCORE[3] qk+2

qk+3

qk

qk+1

qk-2

qk-1

DPk  Rk qk  DP  Rk q s k

s k

q 0 s k

[3] M.M. Dias et al., “Network models for two-phase flow in porous media”. Part I & Part II, Journal of Fluid Mechanics, 1986.

[3]

FluCORE Hydraulic resistances are functions of friction factors. CFD DATA 1000

CFD Resultados CFD

Friction Factor (f)

TRENDLINE Ajuste

fv

100

a f  b Re

10

1

0.1 10

Reynolds 100Number Re

1000

FluCORE

With oil

Without oil (consecutive)

Qg  QN  QS  QW  QE

FluCORE CFD DATA – Nusselt Number Correlations 60

NUSSELT NUMBER (Nu)

Nucanal CFDCFD Nu exterior i with

50

Nu canal interior CFD Nu CFD e with Nu correlação TRENDLINE

Nu

40

Nu  a xad

30

b

20

10

0 0.000

0.001

0.002

0.003

Dimensionless Thermal Length xad

0.004

0.005

FluCORE

FluCORE vs CFD

Dimensionless Flow Rate (q*)

MV Winding - ODAF

Interior ducts FluCORE Interior ducts CFD Exterior ducts FluCORE Exterior ducts CFD Radial ducts FluCORE Radial ducts CFD

Discs (from bottom to top)

FluCORE vs CFD MV Winding - ODAF

Disc Temperature (˚C)

Max. Temp. (FluCORE)

Aver. Temp. (FluCORE)

Max. Temp. CFD

Aver. Temp. CFD

Discs (from bottom to top) Aver. Temp. (ºC)

Max. Temp. (ºC)

CFD

80

99

Model (FluCORE)

79

98

FluCORE – Exp. Validation • 115 /13.8 kV, 30 MVA transformer. • 2 ONAN regimes with radiators at different positions. • 24 Optical Fibres installed from FiberSensing®, Porto, Portugal.

FluCORE – Exp. Validation

Experiment 1 Ambient Temperature Top Oil Temperature Rise Hot-Spot Temp. Rise

HRT

FO

FluCORE

ºC 24 38 55

ºC

ºC 24 35 54

Experiment 2

NA 56

HRT

FO

FluCORE

ºC

ºC

ºC 27

Ambient Temperature

27

Top Oil Temperature Rise

39

Hot-Spot Temp. Rise

57

HRT – Heat Run Test (IEC 60076-2) FO – Optical Fibre Direct Measurements

NA 58

36 57

CONCLUSIONS

1

ANSYS Fluent enhanced the comprehension of the cooling cycles of power transformers.

2

ANSYS Fluent showed good agreement both with flow and heat experiments..

3

ANSYS Fluent minimized experiment costs by delivering important information such as Friction Factor and Nusselt Correlations.

N-1 CFD was the basis of FluCORE which is indeed a powerful engineering tool.

THANKS!!!